Methods for cleaning and descaling titanium



May 10, 1960 H. G. WEBSTER Em 2,936,270

METHODS FOR CLEANING AND DESCALING TITANIUM Filed Nov. l2, 1957 2Sheets-Sheet 1 HUGH G. WEBSTER By ROBERT P. DEVANE @Jagd ATTORNEY H. G.WEBSTER :TAL 2,936,270 METHODS FOR CLEANING AND DESCALING TITANIUM FiledNov. 12, 1957 May 10, 1960 2 Sheets-Sheet 2 INVENTORS HUGH G. WEBSTERM/ZOE'ERT P. DEVANE BY all/WFM ATFORNEY` United States Patent O METHDSFR CLEANING AND DESCALING TITANIUM Hugh G. Webster and Robert P. Devane,Detroit, Mich., assignors to Kolene Corporation, Dptroit, Mich.

Application November 12, 1957, Serial No. 697,761 4 Claims. (Cl.204-141) This application is a continuation-impart of our copendingapplication Serial No. 480,860, led January 10, 1955, now abandoned, andrelates to a method and apparatus for cleaning and/or descaling oftitanium and more particularly to the prevention of burning oftitanium'sheets supported in work-holders while immersed in moltenalkali salt baths.

In the rolling and annealing of titanium from ingots to sheets and thento thinner sheets, it appears that a scale is formed on the hot sheetwhich is a type of oxide of titanium, the exact formula of which isuncertain. It is probably titanium dioxide and upon reaction in acaustic bath the titania (Ti02) is probably dissolved as the titanate(Na2TiO3). It has been found that that scale is readily removed in amolten alkali salt bath of which there are two types, both comprisingpredominantly caustic soda, but essentially differing in that the rstcleans metal merely by dipping in the bath and the second mounts themetal to be cleaned as an electrode in the bath and passes directcurrent therethrough.

The first type is described in U.S. Patent to Webster, 2,458,661. it isa type known in industry as Kolene No. l and, as described in thispatent, includes an oxidizing agent such as a nitrate which, incombination with the caustic, oxidizes and loosens the scale to form asodium titanate solution in the bath. As mentioned in that patent atypical bath composition comprises 1.5 to 3 parts by weight of alkalimetal hydroxide; about 1 part by Weight of alkali metal nitrate; andoptionally about 0.1 to 0.5 part by weight of alkali metal chloride. Aspecific example comprises 2 parts by weight of sodium hydroxide, 1 partby weight of sodium nitrate and 0.5 part by weight of sodium chloride.This type of cleaning bath is unique in that the bath is heated to atemperature in the range of 550 to 1300 F. and the surface of the metalbecomes clean merely by the immersion in the molten bath withoutelectrolysis. For this type of bath, the cleaned metal may be given asubsequent acid bright dip. Generally, for cleaning of ferrous metalssuch bath was used at the lower end of the temperature range generallynot exceeding 900 F.; but for cleaning of titanium metal such bath willusually be used in the upper range, preferably above 750 F., 900 to 1250F. being a preferred range.

'Ihe second type of cleaning bath is described in U.S. Patent toWebster, 2,468,006. That type is known in industry as Kolene No. 4 andas described in this patent, thebath includes an electrolytically activeoxidizing agent, specically sodium aluminate, but no direct actingoxidizing agent of the nitrate type above described. That non-oxidizingtype metal aluminate only during electrolysis serves as an oxygencarrier and oxidizes any free sodium formed at the cathode to sodiumoxide, active to dissolve the metal oxide scale on the work piece to becleaned. The aluminate is continuously regenerated with the dissolutionof the oxidized titanium metal.` Accordingly, it needs to be presentonly in a very small quantity. 'I'he type of cleaning in which suchaluminatc ICC is useful in combination with caustic is exclusively anelectrolytic cleaning in contrast to the viirst type described above.For this purpose the work piece is suspended again in a work-holder andis immersed in the molten salt cleaning solution, the entire assemblybeing made a cathode (and sometimes anode alternately) and the cleaningis elected during the passage of electrolytic current. A typicalcomposition mentioned in the patent comprises about 70 to 90% causticalkali, such as sodium or potassium' hydroxide, about 7 to 13% sodiumchloride and about 0.5 to 5% sodium aluminate. The compositionoptionally includes alkali metal fluoride, and when used, will bepresent in proportion of about 2 to 6%, all proportions being by weight.A typical example of a cleaning bath of the second type may be sodiumhydroxide, about 10% sodium chloride, about 4% sodium fluoride and about1% sodium aluminate; and if the uoride is omitted, such typicalcomposition will be 89% caustic soda, 10% sodium chloride and 1% sodiumaluminate.

The bath of this second type is heated again from about 550 to l300 F.,preferably about 900 to 1250 F. and current at 4 to 6 volts is passedthrough the titanium metal in the work-holder rst as a cathode at acurrent density of about 50 amperes per square foot for a period of l0to 30 minutes and then the current is reversed, making the work pieceand work-holder an anode, and passing current therethrough under thesame conditions for a period of 3 to 10 minutes. The process of currentreversal may be applied alternately several times, 2 or 3, as desired.This cleaned metal may also be given a bright dip in acid such as 10%hydrochloric acid after removal and washing of the molten saltstherefrom.

Other forms of molten caustic baths have been applied to cleaning anddescaling of titanium, such baths, however, giving rise to the sameproblem as solved by the present invention.

It is common practice in such operations to employ stainless steel racksor work-holders for holding the titanium sheets immersed in the bath,stainless steel because in these processes an acid dip or rinse isemployed and stainless steel happens to be the best material developedthus far for work-holder purposes where the process includes aciddipping, inasmuch as the stainless steel is not attacked'dun'ng thatoperation. However, it is found, using such stainless steel work-holderand mounting titanium Ausually as thin sheets in the Work-holder, duringimmersion ofthe titanium in the caustic bath, whether of type 1 or type2 or other caustic baths known in the art, operated with or withoutelectrolysis, the titanium becomes burned at the point and immediatelysurrounding area of contact with the Work-holder, and otherwise becomesdegraded by hydrogen embrittlement n that area. The problem is outlinedmore specifically by reference to the drawings in which, r

Fig. l shows a side view of a typical work-holder in the form of a rackused in the prior art to support titanium sheets with a sheet mountedtherein.

Fig. 2 is an end view of that rack.

Fig. 3 shows a work-holder as in Fig. 1 with a strap bolted thereto andto a titanium sheet mounted therein.

Fig. 4 is an end view of Fig. 3. l

Fig. 5 is a perspective diagram of a rack with a single sheet thereinshowing, exaggeratedly, the actual points of contact between the sheetand the rack.

Fig. 6 is a similar diagram like Fig. 5, including further a strap to bebolted between the rack and sheet.

Fig. 7 illustrates diagrammatically an enlarged detail of experimentallybolting a contact strap to a portion ofa titanium sheet mounted within arack as shown in Fig. 5.

Fig. 8 illustrates diagrammatically an experimental of a plate oftitanium and stainless steel.

Referring iirst to Fig. 8, two 2" square sheets, one v of type 400stainless steel, and the other pure titaniumY metal, the latterperforated for insertion of a l" wide strap comprising an extension ofthe stainless steel plate whereby the strap contacts the titaniuml sheetat its two opposite edges as inserted through the perforation, areimmersed as an assembly in a molten Kolene No. l bath. That bath, asstated above, consists, for example, of 2 parts caustic soda, l partsodium nitrate and 1/2 part sodium chloride. The temperature is raisedgradually. `It is found that at a temperature of 1235 F. the titaniummetal will ignite at the point of contact with the stainless steelstrap, i.e. at 'the points X and X1 of Fig. 8. Oxidation of the titaniummetal from these points will progressively continue at this temperatureand will continue so long as the titanium remains in the bath. Thattemperature is critical in that it is an ignition point. At lowertemperatures, in the immediate area of the point contact, it is apparentthat some reaction, apparently a burningf takes place because the metalappears to be eroded and embrittled, the latter a property of titaniumknown to result from the adsorption of large quantities of hydrogen. Atthe lower temperatures, below the critical point of 1235" F., ignitiondoes not actually take place.

Again, the experiment was repeated applying current to the assembly ofFig. 8 While immersed in a Kolene 4 type of bath by leading electricalcurrent in Contact with the steel plate as a cathode at a currentdensity of 50 amperes per square foot. The bath consisted o-f 89 partsof caustic soda, 10 parts of sodium chloride and l part of sodiumaluminate. The results were the samer indicating that the burning of thetitanium is a function of the bath, the temperature thereof and thedissimilar metals in point contact and is not a function of the presenceor absence of electrolysis applied in the cleaning of the metal.

Thus, the burning appears to be a burning of titanium sheets in spotsemanating from the points where the tita- `nium sheets are in contactwith the stainless steel workholder, such as a rack. It is obvious thatunless the stainless Vsteel rack with the titanium sheet therein ispulled out of the bath very quickly, burning of the titanium metal wouldcontinue with progressive damage.

lFor instance, referring to Fig. 5, a metal rack is constructed of 1/4round stock 316 stainless steel. It is formed of vertical legs 10a,bottom bars 11, top bars 12, and hanger stirrup bars 15, fastenedtogether as by welding into a rack as shown and further described belowin relation to Fig. 1 below, which is similar. A sheet 20a of titanium30 x 36 x .032" is supported in point contact with the rods of the rack.The titanium metal sheet contacted' the rack at 4 points, Y, Y1, Y2 andYa as shown. The'metal and rack as shown in Fig. 5 was immersed in thefirst type of bath(2 parts caustic soda, 1 part sodium nitrate, 0.5 partsodium chloride) at a temperature of 980 F. and thetitanium ignited atall 4 points of contact, Y, Y1, Y2 and Y3; when the metal sheet wasimmersed in the same bath at 850 F. no ignition occurred.V It was foundthat this burning is due to a galvanic action which occurs between thetitanium sheet metal work-piece and the stainless steel rack in which itis supported in the bath. It is further found that the galvanic effectvaries in intensity with the temperature of the bath.

Moreover, although it was rst believed that the current values of thatgalvanic action between the metal work-V piece and the rack would be inthe neighborhood of 100 milliamperes per square inch as could beYcalculated from known electrode potentials of these metals, we found,surprisingly, that the current ilow initially is about ten times asgreat, i.e. 1000 milliamperes per square inch, at the beginning of thecleaning operation and instantaneous conditons may be even higher. Suchhigh current valutcomprise an extremely high density of current at thepoints of contact thereby progressively burning the metal or ig- 4niting it at certain critcial points, depending upon the actualtemperature ofthe bath.

The momentary current flow per square inch was measured between platesof titanium and 300 series stainless steel each 2 square, both 0.032"thick, the plates being arranged as shown in Fig. 8 and immersed in atype I bath. At 700 F. the current flow per square inch thus measuredwas 0.14 ampere milliarnperes); at 820 F. the current flow more thandoubled and was 0.30 ampere k(300 milliamperes); and at 975 F. thecurrent iiow was 1 am-v pere (1000 milliamperes). It is apparent that inthe unitV plate burning experiment described above, the ignition currentat 1235 F. well exceeded 1000 milliamperes per square inch.

Accordingly, We have concluded that it is not 4possible l to conductsuch large current values through these very small points of contactbetween the rack and sheet safely and without burning. We have concludedthat the extremely large current values at the beginning of the cleaningoperation coupled with the small area of contact between the sheet andthe holder causing high current densities at high bath temperatures, isthe primary cause of the burning of the sheet at the start of theoperation.

We have also found that after the cleaning operationV is well under way,polarization sets in and within a few minutes the galvanic current dropsfrom the neighborhood of 300 to well over a 1000 milliamperes per squareinch, depending upon the temperature, down to about 100 milli amperesper square inch, which is insu'icient to causev burning.

According to the present invention We have discoveredv surfacesupported, and at a lower temperature, such asV aY temperature as low as850 F. the minimum average areaf contacted may be reduced to 1/2. Thus,the critical ratio of both exposed surface area of titanium metalsupportedV in a work-holder to the actual contact area with theWorkholder should be 500 units of titanium surface to from 1/2 to 2units of area of contact with the work-holder.

These conditions vary with respect to temperature, requiring about 1/2unit contact area per 500 units of titanium surface at a temperature ofabout 850 F., aboutvv one unit per 500 at a temperature of about 980 F.,and about 2 units of contact area per 500 units, supported attemperatures exceeding about 1100" F. The provision ofA such criticallarge area high quality contact between the rack and the sheet, we havediscovered, resulted in the absence of burning during the cleaningoperation, even at the'start of such operation.

Figs. 1 and 2 illustrate the old style racks commonly used aswork-holders for cleaning of metals of various types, and which allowburning o'f titanium as herein described. Each such rack comprises wireor rods. joined,v

as by welding, to form side bars 10, bottom bars 11, top bars 12,connecting bars 14, and hanger stirrupbars 15. The sheets 20 oftitaniumV are supported with their lower and upper edges 21 and 22 incontact with the bottom bars 11 and top bars 12, corresponding to thepin-point contacts Y, Y1, Y2 and YS, exaggeratedly and diagrammaticallyillustrated in Fig. 5. At A of Fig. 1 is shown an area'of burning at apoint of contact between the sheet 20 and the'bottom bar 11, that areaof burning comprising an eroded discolored area which is substantiallyembrittled.

by adsorption of hydrogen and comprises a greatly damaged area of thesheet, both from the standpoint of the appearance of the sheet which thebath was intended to improve in its initial cleaning, as well as damagein the` physical characteristics of the metal at this point due totheburning and embrittlement. Y

` Example To illustrate the practice of this invention, the sheet 20a,as shown dagrammatically in Figs. 6 and 7, has a strap 30a comprising 3x 24" type 316 stainless steel sheet Welded to the rack, that is, tobars a near the bottoms thereof. The sheet 20w was then bolted at Z,each bolt gripping the sheet through a round stainless steel washer32yielding a contact area therebetween of about 1% square inches, thesheet 20a itself being 30 x 36 x 0.032" and having total surface area of2160 square inches. As described above for Fig. 6 .the sheet 20a andrack were immersed in the Kolene No. 1 bath having the same composition,2 parts caustic soda, l part sodium nitrate and 0.5 part sodiumchloride, heated to 980 F. and ignition occurred immediately despite the11/2 square inchfcontact arearthrough the washer 32, indicating `an arearatio of about 1S00to 1 to be too small at this tempcrature. A secondbolt Z1 was applied thereby increasing the contact area Z and Z1 to 3".No ignition took place with that double area, thereby indicating at thistemperature that the critical surface area of titanium sheet at thework-holder per unit contact area is about 750 to 1, somewhat largerthan necessary. With that enlarged area of contact between the strap 30aand the sheet of titanium metal, not only was there no' burning in thearea of the strap but there was likewise no burning at the upper contactpoints of the titanium plate such as Y and Y2. The experiment of thisexample was repeated in a Kolene No. 4 bath consisting, as stated above,of 89 parts of caustic soda, 10 parts of sodium chloride and 1 part ofvsodium aluminate, and current connected to the rack through the stirruprods to provide a current density of 50 amperes per square inch and at atemperature of 980 F., gave the same burning when no strap was usedaccording to the diagram of Fig. 6, and the same burning with one 11/2square inch surface area washer '32 according to Fig. 7, but all burningwas overcome when at least two washers in 3 square inch area of contactwas used to fasten the strap 30a to the titanium sheet.

It was found that the mounting of titanium sheet with a continuing strap30 as shown in Fig. 3, the rack allowing several sheets to be mounted ina single rack, each separated by strap 30 as may be further noted fromthe end view Fig. 4, all burning of the sheet was terminated at.temperatures slowly raised up to and exceeding I125 0 F. where the areaof contact exceeded 1 unit in 250.

It was found that where the construction of Fig. 3 allowed contact ofone unit in 500 (surface area of titanium) a temperature up to 980 F.produced no ignition or burning; and when the area ratio was one unit in1000 no burning or ignition of the titanium sheet occurred up to 850 F.

In such large size testing three 30" x 36 sheets of titanium wereinserted in a stainless steel wire rack in such a Way that each sheetcontacted the rack at four points only, two at the bottom of each sheetand two at the top of each sheet. The rack was immersed in a Kolene No.1 bath at 900" F. and ignition was observed after 75 seconds. It wasobserved that such ignition had taken place at one of the upper contactpoints on the center sheet of titanium. This sort of experiment wasrepeated several times with generally the same result.

On the other hand, a similar set of sheets and rack were associatedexcept that in this latter embodiment high area, high quality contactbetween the titanium sheets and rack was established by means of 24" x3" strips of stainless steel and 8 clamping bolts with 3 such stainlessstrips being welded to the rack and the eight bolts used to effectexcellent electrical contact between the rack through these strips andthe sheets. The assembly was then immersed in a Kolene No. 1 bath at 980F. and there was no' evidence of ignition despite 150 seconds and moreof immersion. This test was repeated several times with gen- -erally thesame results.

It has also been concluded that inasmuch as the current values lowerthemselves due to polarization shortly after the startlof the cleaningoperation that it is not necessary to' maintain the high quality largearea electrical contact shortly after the start of the operation, butthat instead, much of the area of contact can be removed and satis-`factory results may be obtained, once the operation is well underway andpolarization has taken place, with the conventional type of contact,namely po'or contact over small area and at isolated points. Recognizingthis, we contemplate to provide a releasable device for removing most ofthe area of contact after polarization has set in, in order to' exposethe maximum area of the sheet being cleaned for cleaning purposes.Obviously, if a large area of the sheet is taken up by contact then sucharea is not exposed for cleaning. Therefore, we propose to provide areleasable means which may be operated to insure large area of contactbetween the rack and the sheet at the start of the operation but whichmay be released after polarization sets in so as to reduce the contactarea and to expose more of the sheet for cleaning.

It is expected that in the electrolysis of sodium hydroxide somehydrogen will be released. Where the two dissimilar metals are a pointcontact and gal'vanic current develops in high current density throughthe point contact, copious quantities of hydrogen are evolved at thatpoint which, it is believed, results in the embrittlement of titaniummetal. However, it is found that using the critical large surface areaof contact as hereinabove described, such electrolysis of the causticsoda as may take place by galvanic action, is so widely distributed overthe entire surfaces of the metals that no localized concentration ofhydrogen contacts the titanium and no significant hydrogen embrittlementis found.

Thus, we have discovered that titanium metal supported in a cleaningrack and immersed in a molten salt cleaning b-ath predominantly causticsoda in the range of 500-1300 F., burns at the point of contact of themetal with the rack due to the high density of galvanic current thatmust be carried through such contact points as developed betweendissimilar metals. While -we have disclosed stainless steel as apreferred metal for forming v the rack in view of its resistance tocorrosion in subsequent acid dip, other dissimilar metals such as steel,nickel or copper, useful for lforming the rack which may be used herein,will give rise to the same problem, and such burning will be overcomeIby the present method and apparatus.

Various modifications will occur to those skilled in the art andaccordingly it is intended that the description herein be regarded asillustrative and not limiting except as delined in the claims appendedhereto.

We claim:

l. The method of cleaning the surface of titanium metal comprisingmounting the titanium metal in a lworkholder Iin rm electro-conductivecontact between the work-holder and the titanium metal body over asurface area ranging upward from a minimum of 1K1. to 2 units of area ofcontact between the work-holder and titanium metal surface for each 500units of" exposed titanium surface area, said work-holder being `formedof electroconductive metal having an electrochemical activity less thanthat of the titanium, and immersing the combined Work-holder andtitanium metal mounted therein in a bath comprising predominantly moltencaustic alkaliV maintained at a temperature in the range of 500 to l300F., for a period at least sufficient for initial reaction With the bathcomponents.

2. The method of cleaning titanium metal sheet comprising mounting saidtitanium metal sheet in a workholder in rm electro-conductive contactbetween the work-holder and titanium sheet surface over a surface arearanging upward from a minimum of 1/2 to 2 units of area of contactbetween the work-holder and titanium metal surface for each 500 units ofexposed titanium sur- `7 face area, said work-holder being formed ofelectroconductive met-a1 having an electrochemical activity less thanythat of the titanium, and immersing the combined Work-holder andtitanium metal vsheet mounted Vtherein in a -bath comprisingpredominantly molten caustic alkali maintained at a temperature in therange of about 850 to 1250" F. for a period at least sufficient forinitial reaction With the bath components.`

3. Method as defined inA claim 2 wherein the bath comprises iaboutl/z to3 parts by Weight of alkali metal hydroxide, about 'l part by weight ofalkali metal nitrate and up to about 0.5 part by Weight of alkali meta-lchloride. l

4. The method as defined `in claimr2 `vs fhrerein the bath comprisesabout 70-90% of alkali metal hydroxide, about 7-13% of sodium chloride,about 0.5-5% of sodium aluminate and `about 0-6% of alkali metalfluoride, and the metal is cleane'diby electrolysis While suspended insaid Weils-holder` as an electrode in said bath comprising passingVdirect current through said bath, Work-holder and titanium metalsuspended therein as an electrode. i

References Cited in the file 'of this patentiV UNITED STATES PATENTS1,501,692 Ward Iuly "15, 1'9-24 2,048,854 Dyer July 28, .1936, 2,675,348Greenspan Apr. 13, 1954- 2,852,463 Gutzrner Sept. 16, y1958`

1. THE METHOD OF CLEANING THE SURFACE OF TITANIUM METAL COMPRISINGMOUNTING THE TITANIUM METAL IN A WORKHOLDER IN FIRM ELECTRO-CONDUCTIVECONTACT BETWEEN THE WORK-HOLDER AND THE TITANIUM METAL BODY OVER ASURFACE ARE RANGING UPWARD FROM A MINIUM OF 1/2 OF 2 UNITS OF AREA OFCONTACT BETWEEN THE WORK-HOLDER AND TITANIUM METAL SURFACE FOR EACH 500UNITS OF EXPOSED TITANIUM SURFACE AREA, SAID WORK-HOLDER BEING FORMED OFELECTROCONDUCTIVE METAL HAVING AN ELECTROCHEMICAL ACTIVITY LESS THANTHAT OF THE TITANIUM, AND IMMERSING THE COMBINED WORK-HOLDER ANDTITANIUM METAL MOUNTED THEREIN IN A BATH COMPRISING PREDOMINTALY MOLTENCAUSTIC ALKALI MAINTAINED AT A TEMPERATURE IN THE RANGE OF 500 TO1300*F., FOR A PERIOD AT LEAST SUFFICIENT FOR INITIAL REACTION WITH THEBATH COMPONENTS.